This is a new application in response to PA-99-044, Lab Methods to Assess Responses to HIV Vaccine. The investigator proposes to employ an in vitro challenge assay as a surrogate marker for vaccine-induced protective immunity. The specific aims are to extend his previous work to current vaccine trials and to assess the magnitude of cross-clade resistance. Altogether 10 hypotheses will be tested: 1) vaccination with candidate canarypox vaccines will induce in vitro resistance to HIV-1 MN in >20 percent after two or more immunizations, 2) resistance to HIV-1 MN will correlate with resistance to HIV-1 BaL, 3) resistance to HIV-1 BaL will correlate with resistance to clades C and E R5 primary isolates, 4) resistance to HIV-1 will be highly dependent on CD8+ T cells, and 5) correlated with HIV-specific CTLp to non-envelope epitopes, 6) resistance to R5 isolates will not be fully reversible by beta-chemokine neutralization nor adoptively transferable by soluble factors present in culture supernatants, 7) ADCC will be demonstrable, 8) resistance will occur at the post-integration stage, 9) inducible nitric oxide synthase activity will correlate with resistance , and 10) post-vaccination, preinfection PBMC will not exhibit in vitro resistance.